This invention relates to a process for depositing zinc oxide films on a flat glass substrate.
Due to a number of desirable physical and chemical properties, zinc oxide coatings have been used previously, primarily in connection with UV lasers, piezoelectric devices, gas sensors and transparent conductors for solar cell contacts and flat panel displays.
Growth of zinc oxide coatings by CVD has been reported in the scientific literature. For example, Smith, Frank T. J., “Metalorganic chemical vapor deposition of oriented ZnO films over large areas”, Applied Physics Letters, Vol. 43, No. 12 (1983) pg. 1108-1110, describes a metal organic chemical vapor deposition process for preparing c-axis-oriented ZnO films in a system similar to that which is commercially available for SiO2 deposition. The resulting films are said to be highly uniform in thickness and to adhere to a variety of substrates.
Gerfin and Dahmen in CVD of Nonmetals (W. S. Rees, Jr. ed., VCH Publishers, Inc., New York, N.Y., 1996), chapter 3, pg. 180-185, describe the work of a number of researchers in use of a variety of chemical preparation techniques to form zinc oxide films. Use of dialkyl zinc compounds and various oxygen-containing compounds is discussed.
Gulino, A. et al., “Synthesis and Characterization of Novel Self-Generating Liquid MOCVD Precursors for Thin Films of Zinc Oxide”, Chemistry of Materials, Vol. 12, No. 2 (2000) pg. 548-554, describes the production of Zn(hfa)2.2H2O.polyether adducts and the use of same as precursor materials to form ZnO films on fused SiO2 substrates. One such adduct is Zn(hfa)2.2H2O.diglyme. Characterization of the adducts via spectroscopy and single crystal x-ray diffraction demonstrated that the polyether ligands were not ligated to the zinc center. ZnO films were said to be formed using such adduct in a low-pressure horizontal hot-wall reactor.
Deposition of zinc oxide films has also been described in the patent literature.
U.S. Pat. No. 4,751,149 to Vijaykumar, P., et al. describes a low temperature (200° C. or less) static deposition method for zinc oxide films, utilizing an organozinc compound and water carried in an inert gas. The resulting zinc oxide film is said to have a low resistivity which can be varied by addition of a Group XIII element.
U.S. Pat. No. 6,071,561 to Gordon, R., et al. describes deposition of fluorine-doped zinc oxide films utilizing a chelate of a dialkylzinc, such as an amine chelate, an oxygen source, and a fluorine source. The films produced are said to be highly electrically conductive, transparent to visible light, reflective to infrared radiation, absorbing to ultraviolet light, and free of carbon impurity.
U.S. Pat. No. 6,416,814 to Giolando, D. describes the use of ligated compounds of tin, titanium, and zinc as metal oxide precursor compounds in a method to produce high quality metal oxide coatings when coming in contact with a heated substrate.
U.S. Pat. No. 6,627,765 to Giolando D. describes ligated compounds of tin, titanium and zinc for use as metal oxide precursor compounds which are said not to be extremely reactive, yet maintain the ability to produce high quality metal oxide coatings when coming in contact with a heated substrate.
It would be desirable to form zinc oxide films at essentially atmospheric pressure and to produce them at deposition rates compatible with time-critical manufacturing processes, for example, production of flat glass by the well-known float method. Those skilled in the art have continued to search for a method of producing zinc oxide films meeting the above-noted criteria in order to have available affordable films for optical thin film stack designs.